The present invention relates to an electro-pneumatic transducer for converting an electric signal to a fluid pressure, especially, .a pneumatic pressure, and more particularly to an electropneumatic transducer including as a transduction element a nozzle flapper comprising an electrostrictive element, and incorporating a pilot valve for increasing an output gain.
Heretofore, torque motors have widely been used as electropneumatic transducers for converting electric signals to pneumatic pressures. The torque motor has a coil supplied with a current to produce a rotative force commensurate with the supplied current, the rotative force being converted as a nozzle flapper, a pilot valve, etc. to a pneumatic pressure. Normally, the current supplied to the torque motor is a direct current ranging from 4 mA to 20 mA.
Where a control device such as an electro-pneumatic transducer employing the torque motor, the control device is more resistant to mechanical vibrations and other disturbances and stabler in performance if the motor generates a greater torque.
In view of recent demands for smaller and lighter control devices, it has become an important task to make torque motors smaller in size. In general, however, the smaller the torque motor, the smaller the torque produced dependent on the supplied current, and hence the less resistant to mechanical vibrations. Under some conditions in which the control device is intended to be used, it would be technically impossible to employ the torque motor.
The inventor has made efforts to reduce the size and weight of a torque motor while making it more resistant to vibrations and impacts. As a result, the inventor has found electrostrictive elements to be of much interest as a transduction element for converting an electric signal to a pneumatic pressure. Although there are different shapes and materials available for electrostrictive elements, the general arrangement is known as the bimorph-type electrostrictive element in the form of a thin rectangular plate disposed as a cantilever with one end fixed and the other end free. When a voltage is applied between the electrodes, the free end of the bimorph-type electrostrictive element is slightly displaced. By constructing a nozzle flapper of an electrostrictive element itself, therefore, a voltage change can easily be converted into a nozzle back pressure. One known nozzle flapper employing an electrostrictive element is disclosed in PCT/SE80/00057 as "A signal converting unit intended to be incorporated in a pneumatic control system". With the above arrangement, the torque motor conventionally used as the transduction element can be replaced with the electrostrictive element. In case the electrostrictive element is sized as 10 mm.times.20 mm, for example, and has a thickness of about 0.6 mm, the nozzle flapper has a mass that is negligibly small as compared with the torque motor, and is highly resistant to vibrations and impacts.
While the nozzle flapper constructed of the electrostrictive element is of an improved vibration and impact resistance capability, some problems still remain to be solved if it is to be combined with a pilot valve to make an electropneumatic transducer as a final product.
More specifically, since the electrostrictive element is generally displaceable only slightly in response to a change in the voltage applied thereto, the applied voltage should be increased and the required electric circuit should be complicated if a relatively large displacement is to be produced by the electrostrictive element. If the electrostrictive element were displaced to a large degree, then it would be less durable in use.